In an electrical discharge machine for machining a workpiece by electrical discharge occurring in a gap between an electrode and a workpiece (hereafter referred to as the electrode gap, as necessary) while changing the relative position of the electrode and the workpiece, it has been the practice to set a reference value for accurately ascertaining the aforementioned relative position so as to obtain high geometric accuracy. Conventionally, to set such a reference value, a positioning apparatus is known in which the electrode and the workpiece are brought into direct contact with each other, and this position of contact is set as the reference.
Referring to FIG. 10, a description will be given of this positioning apparatus. FIG. 10 is a block diagram illustrating a conventional electrical discharge machine in its entirety. In FIG. 10, the electrical discharge machine has as its main components an electrode 101 for machining; an X-axis moving means 103, a Y-axis moving means 104, and a Z-axis moving means 105 for relatively moving a workpiece 102 with respect to the electrode 101; a power supply 106 for machining for applying a voltage pulse between the electrode 101 and the workpiece 102; an NC control means 107 for imparting instructions on the position to the X-axis moving means 103, the Y-axis moving means 104, and the Z-axis moving means 105; and a contact detecting means 108 serving as a positioning apparatus for detecting the state of contact between the electrode 101 and the workpiece 102.
Referring to FIG. 10, a description will be given of the positioning of the electrical discharge machine arranged as described above. First, in the setup work prior to machining, the X-axis moving means 103, the Y-axis moving means 104, and the Z-axis moving means 105 are operated in accordance with the instructions on the position from the NC control means 107, and the fact that the electrode 101 has contacted the workpiece 102 is detected by the contact detecting means 108 so as to set a reference value. Since the coordinates of the relative position of the electrode 101 and the workpiece 102 can be detected, high-accuracy machining is possible.
However, in a case where a contact portion of the electrode 101 is very small, since the electrode 101 and the workpiece 102 are brought into contact with each other, there has been a first problem in that the tip of the electrode 101 can be broken by the impact.
In addition, to effect very fine machining, it has been the practice to subject a rod electrode having a diameter of several tens of microns or thereabouts as the electrode 101 to dressing forming by means of reverse discharge by using a block electrode and a wire electrode, and to effect fine machining by means of the formed electrode 101, but there has been a problem similar to the aforementioned first problem.
In addition, the actual depth after machining subsequent to the positioning is one in which the amount of discharge gap is added to the amount of feeding of the haft during machining, as shown in FIG. 11. Accordingly, the amount of feeding is a value in which the amount of discharge gap is subtracted from the depth after machining. However, since data on the amount of discharge gap must be acquired in advance for each electrical condition, and the value varies due to the working-fluid jetting pressure for the electrode gap, the electrode-gap sludge, and the like, there has been a second problem in that the above-described conventional electrical discharge machine is unsuitable for high-accuracy machining.